1. Field of the Invention
The present invention relates to a digital mobile communication system which is used for public business digital mobile communications and the like.
2. Description of the Prior Art
Recently, information communication technologies have rapidly developed. In particular, mobile communication technology in the radio communication field has remarkably rapidly developed, as has optical communication technology in the wire communication field.
Mobile communications are commonly categorized into commercial communications and non-commercial communications. Among the former communications, there are typically cordless telephones and portable telephones. Among the latter communications, there are typically taxi radio communication, MCA (Multi-Channel Access) land mobile radio communication systems, and public business digital mobile communication systems.
If there arises a disaster, the public business digital mobile communication system considerably serves for transmission of information on the disaster, in contrast with the other communication systems or networks which might malfunction. Accordingly, for communications during a disaster, which are mainly performed by a base station and mobile stations communicating therewith a public business digital mobile communication system, is essential.
Incidentally, in a conventional mobile communication system, e.g., the public business digital mobile communication system, the communication among the mobile stations requires the base station. More specifically, the mobile stations always communicate with each other via the base station, A plurality of base stations are arranged at given distances between them. Each base station has a base station zone to control communication therein. Those base stations are controlled by a control station.
Communication between a mobile station and another mobile station requires a path via two base stations and one control station. For example, there is needed a path like the former mobile station - a first base station - a control station - a second mobile station - the latter base station. Even though the former mobile station and the latter mobile station exit simultaneously in the first base station zone, the communication therebetween requires a path like the former mobile station - the first base station - the control station - the first base station - the latter mobile station. In other words, the control station serves for any communication among any base stations.
In general, base stations are positioned on tops of mountains in order to give larger base station zones, which preferably do not require the presence of any person. In consequence, the base stations require a simple structure immune to a failure or a fault.
In a disaster, however, in the case where there occurs a break in the line between a base station and a control station, even though the base station functions regularly, mobile stations simultaneously positioned within the base stations zone of the base station fail to communicate with each other.